I think that 300W Powersupply would be more than enough, TBH, especially if you never plan on adding a discrete GPU. Even there, some of the low end GPUs that can Xfire with Trinity have a thermal ceiling of 75W anyways, so you should still be 'safe'.

Make sure you get a downdraft cooler that fits in that Sugo case. The stock AMD HSF are LOUD!!!!